Hi Janet You can use vertically place 4X4's to temporarily support the headers over the columns without any problem.....but the main concern is if two headers butt above the any of the columns, which if they do will mean that you will have to put temporary supports on each side of that column when replacing it. Do the first floor first, using screw type house jacks (5 to 10 ton each) to unweight (not lift) the header so the old column can be removed and replaced. The other consideration is if the space under the first floor is adequately supported for using the house jacks. If you're over a concrete foundation, should be no problem. But if over free spanned joists, you will have to put temporary supports down there also.